Pressure measurement device, in particular for an infusion apparatus

ABSTRACT

A device for measuring the pressure in an infusion tube (12), including a pressure sensor (20) and a housing (14) comprising at least one chamber (16) with one side defined by a deformable membrane (18) contacting the wall of the infusion tube, and containing said sensor. A fluid (24) is provided between the membrane (18) and the sensor (20). The fluid is non-liquid and has a Poisson ratio of at least 0.49 as well as an instantaneous modulus of elasticity of under 10 MPa so that the sensor response curve is linear. The device may be used in an infusion apparatus, particularly a portable infusion apparatus.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention concerns pressure sensors and in particular theirapplication to a pressure monitoring device in an infusion apparatus.

2. Description of Prior Art

Infusion pumps are used to inject medicamentous substances into apatient's body, usually intravascularly, but equally by otherappropriate access channels.

It is useful to be able to monitor the flow pressure of the fluid as itcomes down from the infusion pump, so as to detect any possible anomalyin the apparatus in operation. Indeed, various incidents may occur: thepump may race or adversely stop working, the catheter through which themedicamentous substance is injected may progressively close up withblood deposits, the infusion tube may be plicated, a leak may occur atany point in the infusion tube, etc . . .

Switches are currently used to monitor flow pressure in an infusiontube, but they work as a two-way system (on or off) and are activated bya substantial variation in pressure in relation to the nominal operatingpressure. Such devices are not very precise, with measurement precisionbeing of the order of 50% of the value of measured pressure. They areoften used simply to control the shutting down of the motor driving thepump when the pressure exceeds a certain level.

Other more sophisticated devices are also known, such as those describedin patents U.S. Pat. No. 4,446,344 and U.S. Pat. No. 4,431,425. Thedevices described in said patents comprise switches capable of not onlystopping the motor when the pressure is too high but also restarting themotor once the pressure has descended to an acceptable value. However,there still remains the disadvantage of insufficient precision.

A device specially designed for the medical field is described in patentDE-A-2,461,424. Said device consists of a chamber filled with glycerinewhich transmits pressure to a sensor, said pressure being exerted on thewall of the sensor's chamber. Such a device with liquid (glycerine) as apressure transmission fluid is difficult to use for measuring pressurein an infusion tube. Indeed, so that there may be an instantaneousreaction to variations of pressure in an infusion tube, it is necessaryfor the transmission fluid response to be linear, and such is not thecase of liquid.

SUMMARY OF THE INVENTION

The purpose of this invention therefore is to propose a pressuremeasurement device which enables continuous and sufficiently precisemeasurement of fluid contained in an infusion tube.

Another purpose of the invention is to create a pressure measurementdevice in an infusion tube using a sensor with a linear response curvewhich reacts to variations in pressure.

The object of the invention therefore is a pressure measurement deviceconsisting of a pressure sensor, a housing comprising at least onechamber with one side defined by a deformable membrane contacting thewall of the infusion tube and containing said sensor, with a fluidbetween the membrane and the sensor. Said fluid is non-liquid and has aPoisson ratio of at least 0.49 as well as an instantaneous modulus ofelasticity of under 10 MPa so that the sensor response curve is linear.

In conformity with another characteristic, the fluid is a silicone gel.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Other characteristics and advantages of the invention shall becomeapparent in the following description given with references to theappended diagram which is presented purely as an example and provides across-section, schematic view of a sensor according to the invention,associated with an infusion tube.

In the diagram, part of an infusion apparatus is schematicallyrepresented comprising a pump with part 10 of its housing from which aninfusion tube 12 is supplied which may be made of silicone, for example.A device according to this invention is used to continuously measureflow pressure in an infusion tube.

Said device comprises a housing 14 defining at least one chamber 16 withone of its walls formed by a deformable membrane 18. Said membrane maybe made of elastomeric material or metal. Its elasticity may be derivedfrom the nature of the material used or from the form given to themembrane. Said membrane may thus be formed by metal expansion bellows.It may be flat, or have a cylindrical concave form to fit an infusionline, or slightly rounded on the outside to increase the sensitivity ofthe measurement device. It is fixed to the housing by any appropriatemeans.

A standard piezoelectric sensor 20 is situated on the floor of thechamber 16 and must be of a suitable type, ie. resistive or capacitive,for measuring absolute or relative pressure.

By way of an example, such a sensor might include a silicon plateassociated with an electrical Wheatstone bridge circuit, comprising, forinstance, five connection conductors 22.

A fluid 24 is situated between the membrane and the piezoelectricsensor. The role of said fluid is absolutely essential as it maintainsthe transmission of pressure from the membrane 18 to the piezoelectricsensor 20. An instantaneous reaction is essential to any variation ofpressure in the infusion tube which might, among other things, put thepatient at risk. Said instantaneous reaction requires a linear sensorresponse, as any deviation in the response curve is difficult tointerpret with a non-linear response. So as to satisfy said linearityrequirement, the fluid in the chamber must be incompressible or nearincompressible with a very low modulus of elasticity. Such a substance,which cannot be a liquid, must therefore have a Poisson ratio of no lessthan 0.49, an instantaneous modulus of elasticity of under 10 MPa, and aviscosity of the order of several hundred centipoises.

An example of such a substance might be a silicone gel such as thatapplied to a patient's skin to transmit an ultrasound signal from atransducer through the patient's body. As examples we cite "Silicone gelSystem Q7-2218" by Dow Corning and "Silicone gel System Q7-2167" by thesame company, associated with "Silicone gel System Q7-2168".

Such a device is particularly suitable for use with a portable infusionpump given the slightness of its size and weight and the fact that itenables continuous measurement, which is precise, reliable and linear,of the changes in pressure coming down from the pump.

Precision may be of the order of 1% of the value of the measuredpressure. The measurement is linear due to the use of a piezoelectricsensor and of a material suitable for transmitting variations ofpressure from the membrane to said sensor.

The device according to this invention enables detection of any absenceof liquid in the infusion tube, in the event of a leak or an emptycontainer.

the device according to this invention enables monitoring of pressurevariations in the infusion apparatus. In the event of progressiveobstruction due to blood deposits in a vascular catheter, the deviceaccording to this invention enables detection of the correspondingincrease in pressure. Warning is thus given before the catheter iscompletely obstructed enabling timely cleaning of the tube. In this way,explantation and replacement of the catheter is avoided.

Such a device may be associated with an electronic control unit whichoperates the overall functioning of the pump. The information providedby the sensor is then used to control and adapt the flow from the pump,something that standard two-way switches do not do.

Among the other advantages of this device, it may be noted that thesensor is not in direct contact with the fluid contained in the infusiontube.

I claim:
 1. A pressure measurement device for measuring the pressurewithin an infusion tube (12) as a medicamentous substance flows throughthe infusion tube by operation of an infusion pump that is connected tothe infusion tube, said device comprising;a pressure sensor (20), ahousing (14) comprising at least one chamber (16) with one side of saidchamber being defined by a deformable membrane (18) contacting a wall ofthe infusion tube, and said chamber containing said sensor at a locationwithin said chamber that is spaced from said membrane, and a fluid (24)within said chamber between said membrane (18) and said sensor (20),said fluid being nonliquid, said fluid having a Poisson ratio of atleast 0.49, and said fluid having an instantaneous modulus of elasticityof under 10 MPa, so that said sensor has a linear pressure responsecurve.
 2. The device according to claim 1 wherein said sensor is apiezoelectric sensor.
 3. The device according to claim 1 or 2, whereinsaid fluid (24) has a viscosity of the order of several hundredcentipoises.
 4. The device according to claim 1, wherein said fluid is asilicone gel.
 5. The device according to claim 4, wherein saiddeformable membrane (18) is cylindrical convex in form.
 6. The deviceaccording to claim 4 wherein said fluid (24) has a viscosity of theorder of several hundred centipoises.
 7. The device according to claim 6wherein said sensor (20) is a piezoelectric sensor.